webadmin: select highest version in new cluster popup

While creating a new cluster, select the possible highest
version depending on the selected data center.

Change-Id: I224b3458e3d4365fa12ea63831952f01e3a5b2f5
Bug-Url: https://bugzilla.redhat.com/975055
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>